The present invention relates to a system for improving the RFI noise immunity of electronic circuits and, more particularly, to a system for shielding the engine electronics used on internal combustion engines from spark ignition RFI, particularly where the electronics are of necessity located in close proximity to the RFI source.
It is well known that high voltage ignition systems are one of the worst sources of radio frequency interference (RFI). In many internal combustion engines, electronic control modules are utilized to control such functions as spark ignition and fuel injection. When utilized in compact design environments, such as for example in outboard boat motors, the electronic control modules are often crowded under an enclosing cowling and located unavoidably close to a very powerful source of RFI.
For example, the second side of an ignition coil can develop 15,000 volts immediately before spark plug gap breakdown and discharge. In some cases, the voltage may even reach levels as high as 30,000 volts. The wave front which is generated by an almost instantaneous voltage drop from 15,000 volts to essentially zero volts results in the radiation of electromagnetic interference signals over a wide band. Typically, however, there is a concentration of energy at frequencies in the range of approximately 150 megahertz. These instantaneous bursts of high frequency RFI can induce voltages in nearby conductors, including the wires to and from a nearby electronic control module. Such voltage transients may be induced in nearby wires as a result of the high currents at high frequencies accompanying the spark discharge or the stray capacitance coupling to a high tension lead could also cause a voltage change in a closely positioned wire. Thus, a 15,000 volt transient on the high tension leads of an ignition system might easily induce a 1500 volt transient in a lead to or from an engine electronic control module which includes the usual PC board construction. In a typical lead wire having a typical characteristic impedance of about 150 ohms, a 1500 volt voltage transient will be accompanied by a 10 amp current transient and an injection of a current of that magnitude into a circuit is likely to cause a potential change at that point in the circuit relative to other parts of the circuit.
It is, of course, known to use suppressor spark plugs to reduce RFI noise from spark ignition systems. However, because suppressor plugs may occasionally not function as intended, a single unsuppressed firing could generate a very strong RFI signal and cause a serious electronic circuit malfunction or failure. Also, in high performance outboard motors using fast rise time capacitive discharge ignition systems, the typical automotive resistor-type suppressor spark plugs cause a drop in power output plus a noticeable reduction in the smoothness of engine idle. For this reason suppressor plugs are not universally used in outboard motors.
RFI transients are particularly troublesome in digital electronic systems. A voltage transient as low as 5 volts is normally sufficient to change a bit in a digital system from digital zero to one or vice versa. Furthermore, a dropped or added digital bit may become temporarily locked in a digital circuit if it is not immediately self-correcting. Thus, an improper logic state may remain locked in for a significant period of time, resulting in a control malfunction. Analog circuits are also susceptible to malfunctions as a result of transient RFI voltages, for example, timing circuits may be caused to operate out of proper sequence by a transient voltage. Ordinarily, however, analog circuits tend to be immediately self-correcting. Thus, the problems caused by RFI are generally much more serious in a digital circuit.
Electronic modules are often shielded against RFI by enclosing them in metal housings. Such metal housings do tend to protect the printed circuit board of the module inside the housing from externally generated electromagnetic fields. Nevertheless, electronic modules used on internal combustion engines still tend to experience problems with spark plug RFI. One of the primary sources of trouble is in the wires entering the module through the housing which pick up and conduct into the housing the strong transient voltages induced from the firing of the spark plugs.
It is known to use RFI suppressing by-pass capacitors to prevent high frequency voltage transients on the wires from reaching the inputs of amplifiers, logic gates, etc. on the PC board inside the housing. Typically, however, such by-pass capacitors are located to merely shunt the RFI voltage transients directly to the ground network on the PC board. These transient voltages are accompanied by strong transient currents and shorting the transient voltage directly to the circuit board ground, via a by-passing capacitor, will result in an instantaneous injection of an electric charge at that point. As a result, the potential at the point of injection will also instantaneously change relative to other parts of the ground network. Should the potential difference be high enough, e.g. about 2.5 volts or more, a logic signal from one digital device located at one point on the ground network may be read as at an opposite state by another digital device located at another point on the ground network, which is at a different relative potential.
Among the wires typically entering the control module through the housing is a ground wire from the engine block. Often, the ground wire extends directly to a connection on the PC board ground network. However, transient high frequency electrical charges induced in the ground wire are then carried directly to the PC board.
The metal housing itself may also experience induced voltages, resulting in a potential difference between opposite ends of the housing. If all four corners of the PC board inside are grounded to the housing, as is often done in conventional design, the potential difference across the housing could also introduce disturbances to the PC ground network.
Also, shielded cables are frequently used to bring low level signals into an electronic module. If the conductive shield on such a cable is allowed to pass directly to the PC board ground network, there will be another potential source of induced transient voltages being transmitted directly to the PC board ground network.
Thus, attempts to provide RFI suppression and shielding for electronic circuit modules has often been ineffective, particularly in high RFI environments where design requirements dictate close proximity between the RFI source and the electronic circuitry, either digital or analog. The generally-accepted assumption that the ground network for a PC board is always a true system ground, even under the varying influences of RFI transients, is believed to be the major source of the problem.